1. Field of the Invention
This invention relates to a moving body position detecting device and more particularly to a detecting device adapted for detecting the moving position of an optical element such as a lens or the like in an optical apparatus such as a camera.
2. Description of the Related Art
Cameras are arranged these days to automatically perform both power varying and focusing actions, which are carried out by shifting the positions of optical elements such as lenses. Hence, cameras are provided with devices for detecting the positions of the optical elements to be used for automatically performing these actions.
The position detecting devices arranged in the conventional cameras to detect, for example, the position of a power varying optical system include two types.
(a) Sliding Contact (Coded) Type Detecting Device
In the device of this type, a flexible printed circuit board on which a coded conductive pattern is formed is secured to the outer circumferential part of a zoom ring of the lens barrel of the camera or the like. A brush (conductive sliding contact pieces or terminals) arranged to come into sliding contact with the conductive pattern is secured to an immovable part of the lens barrel. When the zoom ring is rotated, a voltage which corresponds to the position of the brush on the conductive pattern is detected as a binary coded signal.
An example of the conventional detecting device of this type is described with reference to FIGS. 5 and 6 as follows. Referring to FIG. 5, a zoom lens barrel 1 is arranged to support a zoom lens and to be helically movable in the direction of an optical axis, i.e., to be axially movable while rotating. A motor 2 is arranged to drive the lens barrel 1. A reduction gear 3 has an output gear 3a. A zoom driving ring 4 is fitted on the lens barrel 1 through a helicoid and is arranged to be rotatable only. A ring gear 4a is fixed to the outer circumferential part of the zoom driving ring 4 to be constantly in mesh with the output gear 3a of the reduction gear 3. A pattern plate 5 which is a flexible printed circuit board or the like is secured to the outer circumferential surface of the zoom driving ring 4 and has a conductive pattern 5a formed on its surface as shown in FIG. 6. A brush 6 is secured to a stationary member (not shown) and is arranged to be in sliding contact with the conductive pattern 5a. As shown in FIGS. 5 and 6, the brush 6 consists of five conductive sliding contact pieces (or conductive terminals) 6a. One of the five contact pieces 6a is arranged to be a common contact piece (a common terminal) and to be constantly in contact with the conductive pattern 5a. Other conductive sliding contact pieces 6a are arranged in such a way as either to come into contact or not to come into contact with the conductive pattern 5a. A detecting circuit which is not shown is arranged to detect the output of the conductive sliding contact piece 6a which is in contact with the conductive pattern 5a as a digital signal "1" and that of the conductive sliding contact piece which is not in contact with the conductive pattern 5a as a digital signal "0". In other words, the rotating position of the zoom driving ring 4 can be detected as a binary signal of four bits by using the four conductive sliding contact pieces 6a excluding the common contact piece 6a. Thus, since the output of each of the four conductive sliding contact pieces 6a is in a state of either "1" or "0", the number of possible combinations of the output states of the four conductive sliding contact pieces 6a is 2.sup.4 =16. Hence, 16 positions can be detected by the detecting device of this type. However, the conventional detecting device of this type has the following drawbacks.
(1) The signal outputted from the device would remain unchanged if the states of contact and noncontact do not vary even when the relative positions of the conductive pattern and the conductive sliding contact pieces vary. Under such a condition, detection values are obtainable only in an intermittent manner and continuous feedback control becomes impossible since detection values cannot be continuously obtained. In this case, therefore, zoom control is performed virtually in a stepwise manner.
(2) The number of detecting positions must be increased for obtaining continuous detection values. In order to increase the detecting positions, the number of the conductive patterns and that of the conductive sliding contact pieces of the brush must be increased. Then, such an arrangement necessitates an increase in width of the conductive pattern plate and the brush, and thus results in an increase in cost and space required.
(3) Since positional changes cannot be detected continuously, use of a highly advanced mode of control, such as predictive control, is hardly possible.
(4) Since there are errors in mounting the conductive pattern plate and the brush in addition to some manufacturing errors, position detecting signals tend to deviate from actual lens positions. Therefore, the mounting positions of the conductive pattern plate and the brush must be adjusted, particularly, in a case where a lens position must be detected with absolute accuracy, such as an end position obtained with the lens barrel retracted, a wide-angle end position or a telephoto end position. However, adjustment work on their mounting positions requires time and labor and thus causes an increase in cost. Besides, since the width of each of stepped zones at the lens positions mentioned above is determined by the accuracy of the conductive pattern, it is adjustable only at a single point.
(b) Noncontact (Encoder) Type Detecting Device.
In the case of the noncontact type detecting device, the conductor pattern plate 5 is not applied to the outer circumferential surface of the zoom driving ring 4 as shown in FIG. 5. Instead, a known pulse plate (coding plate) on which a bright-and-dark pattern is formed is mounted on a shaft of the gear 3a or the like. A noncontact type detector such as a photo-interrupter which generates a pulse signal according to the bright-and-dark pattern is mounted on a fixed member. The rotation of the zoom driving ring is detected in a noncontact manner through the rotation of the gear.
The detecting device of the type described in the paragraph (b) above is capable of detecting the amount of rotation of the zoom driving ring more finely than the detecting device of the type described in the paragraph (a). However, the detecting device of the paragraph (b) also has the following drawbacks.
(1) The device consisting in combination of the known pulse plate and the photo-interrupter is in general an incremental encoder and is, therefore, incapable of detecting an absolute position, although the device can detect amounts of changes in position. The device thus necessitates use of at least another detecting means for detecting a datum position.
(2) If a zooming action is repeated within an area without passing through the above-stated datum position, an accumulated error becomes too large for accurate lens positioning.
(3) The lens must be moved to the above-stated datum position immediately after replacement of the battery of the camera or upon return from an abnormal state. If not, lens positioning becomes very inaccurate thereafter.
(4) Since the pulse plate is mounted either on a gear or a shaft of a power transmission system, it is impossible to detect the true position or true amount of movement of the lens. In other words, there is some mechanical play in the gear train or the like of the power transmission system and the signal outputted from the detecting device includes an amount of error which corresponds to the amount of the play. However, it is difficult to remove the error from the signal. When the lens position is changed by a force applied from outside, therefore, the result of lens position control thereafter includes an error, making accurate positioning impossible.
(c) Known devices for detecting the moving amount of a focusing lens, not a zoom lens, include a device disclosed, for example, in U.S. Pat. No. 33,185. This focusing-lens-moving-amount detecting device is arranged to detect the moving amount of a focusing lens by having a brush in sliding contact with a resistance body arranged along the outer circumferential surface of a focus ring to give distance information and by detecting a voltage which corresponds to the position of the brush on the resistance body. This detecting device, however, also has the following drawbacks.
(1) According to the electrical arrangement of the detecting device, it is impossible to obtain a detection value in linear proportion to the amount of movement of the brush relative to the resistance body.
(2) Since a power supply must be connected directly to the resistance body, it is necessary to have the power supply mounted on the rotating focus ring or to arrange the focus ring to include an electric connection part. However, the arrangement of having the electric connection part or the power supply mounted on the side of the rotating body not only results in a complex construction but also tends to cause electrical troubles.
To eliminate the intrinsic drawbacks of the conventional position detecting devices described, a novel position detecting device has been proposed in Japanese Laid-Open Patent Application No. HEI 5-208366. The device proposed is arranged to use a linear potentiometer as shown in outline in FIGS. 7 and 8. The arrangement is as described below with reference to FIGS. 7 and 8.
FIG. 7 is an exploded oblique view of the linear potentiometer. Referring to FIG. 7, a base plate 20 is provided for a circuit board 21. The circuit board 21 is provided with conductive bodies 21a and 21b and a resistance body 21c, which are formed on the surface of the circuit board 21. Three terminals 22 are connected respectively to the conductive bodies 21a and 21b and the resistance body 21c. A brush 23 consists of two sliding contact pieces 23a and 23b, which are arranged to slide respectively over the conductive body 21b and the resistance body 21c. The sliding contact pieces 23a and 23b are short-circuited to each other and are mounted on a sliding member 24. The sliding member 24 is provided with a protruding part 24a. The protruding part 24a is slidably inserted into a slot part 25a formed in a casing 25 and is arranged to be movable in the longitudinal direction of the slot part 25 relative to the casing 25, i.e., in the longitudinal directions of the conductive bodies 21a and 21b and the resistance body 21c. The casing 25 is connected to the base plate 20 in such a way as to have the sliding member 24, the sliding contact pieces 23 and the circuit board 21 sandwiched in between the casing 25 and the base plate 20.
As shown in FIG. 8, the three terminals 22 are connected respectively to the ends of the conductive bodies 21a and 21b and the resistance body 21c and also to points (a), (b) and (c) of a detecting circuit (an equivalent circuit of the linear potentiometer). A resistor R is a total resistance R.sub.FG between the two ends F and G of the resistance body 21c. A constant voltage source 60 (having an output voltage V) is connected to the resistor R (i.e., between the terminals 22a and 22c) so as to apply the constant voltage V across both ends of the resistor R. The terminal 22b is connected to a moving terminal which is arranged to slide over the resistor R.
With a distance between the two ends F and G in the arrangement described above assumed to be L, a voltage v to be detected at the point (b) of the detecting circuit (i.e., at the terminal 22b) when the sliding contact pieces 23a and 23b are in contact with the resistance body 21c and the conductive body 21b at points D and E located at a distance .alpha. from the point F can be expressed as: EQU v={(L-.alpha.)/L}V=(1-.alpha./L)V
When the sliding member 24 and the sliding contact pieces 23a and 23b move from the positions D and E along and relative to the resistance body 21c and the conductive body 21b as much as a distance X to reach positions D' and E', the voltage v' which is detected at the point (b) of the detecting circuit can be expressed as: ##EQU1##
In other words, since the voltage v' detected at an arbitrary point is in linear proportion to the moving distance X, the positions of the sliding contact pieces 23a and 23b (which represent the position of a moving body) can be directly detected. The moving body position detecting device of the kind using the linear potentiometer described above thus has the following advantages:
(1) The absolute positions of the moving body can be continuously detected.
(2) The device requires use of only three electrical connection pins, which are fewer than the pins necessary for the conventional detecting device. (Five connection pins are required for the above-stated coded type detecting device, assuming that it is of a 4-bit, 16 position type; and four connection pins and the datum position detecting means plus connection pins for the datum position detecting means are required in the case of the above-stated encoder type detecting device).
However, the above-stated device disclosed in Japanese Laid-Open Patent Application No. HEI 5-208366 also has the following problems which remain to be solved.
(1) Since the linear potentiometer is adapted for detecting a moving body making a linear motion, use of the device in detecting a rotating position of a rotary body necessitates a motion converting mechanism for converting the rotating motion into a linear motion. As a result, space necessary for the detecting system of the device increases and the cost of the device as a whole also increases by the cost of the motion converting mechanism.
(2) In a case where the brush 23 is mounted on the moving body while the circuit board 21 is mounted on a stationary body, the electrical component part is located on the side of the stationary body. Therefore, in such a case, there arises no problem with respect to electrical connection as an electrical connection part for connecting the resistance body and the conductive body on the circuit board 21 to the power supply and voltage detecting means does not move. However, in the case of such an optical apparatus as a camera or the like, the arrangement of mounting a protruding matter such as the brush on the moving body or on a rotating body not only necessitates a large space around the moving or rotating body but also causes a mass unbalance of the moving body and an increase in size.
(3) In a case where the brush 23 is mounted on the stationary body while the circuit board 21 is mounted on the moving body, conversely to the above, the power supply and the voltage detecting means which are to be electrically connected to the resistance body and the conductive body on the circuit board 21 must be also mounted on the moving body. This arrangement, however, causes an increase in mass of the moving body and is not desirable also in respect of the design of mechanism of the device.
Further, if the circuit board 21 is mounted on the moving body while the power supply and the voltage detecting means are mounted on a stationary body, the electrical connection part for connecting the resistance body and the conductive body on the circuit board to the power supply and the voltage detecting means moves as the moving body moves. That arrangement is, therefore, also not desirable.